Where's my `Array#sum`?

I am curious about why

[1,2,3].max

works as expected (returning

6

), but

[1,2,3].sum

gives me an undefined method error. Either my installation is
bad in some strange and subtle way because my experience seems to conflict with
the documentation, or there's something I'm failing to grasp (perhaps far more
likely. :) ). However, I'm not sure how to determine which it is. I fear I've
sacrificed brevity in pursuit of clarity in my attempt to describe this, and if
there's a better place to ask or a better way to ask, I'm happy to do so, and
delete this question. But this is really bugging me (My comments marked with

##

):

Array is supposed to include Enumerable, according to common sense, my prior
understanding, and the answers to this
question.